Generators of DC voltage from a Digital to Analogue Converter (DAC) are known. A Central Processing Unit (CPU) controls the converter by introducing a digital value of determined precision into an input register. If the value is expressed on 8 bits, 256 values are possible and as a result the precision will be 1/256, that is 0.4%. If for example, the voltage varies from 0 volts to 10 volts, the voltage difference from one value to another is 0.04 volts. Commonly found on the market are DACs with 16 bit registers, the central processing unit can introduce 65,536 different values and the precision goes to 0.0015%. Taking the same example as previously with a voltage varying from 0 volts to 10 volts, two consecutive values present a difference of 0.15 millivolts.
The price of the DACs increases as the precision is increased and the conversion time is more rapid. If a rapid conversion time is not required, the Pulse Width Modulator (PWM) signals enable supply of an analogue voltage from a digital magnitude. A PWM signal is a periodic digital signal for which the period at “1” of the signal is variable within the total period. The period of the signal at “1” is called the cyclic ratio. A base clock supplies the base period. Take as an example the PWM signal generator having an 8-bit program register in which the value “1” is programmed, the digital signal obtained is at “1” for a single base pulse and at “0” during the following 255 base pulses. The digital output of the PWM signal is connected to a RC integrator network that smoothes out the variations. A continuous value is thus obtained that can be amplified to control a motor for example.
This type of assembly is commonly found in regulation systems, to modulate the brightness of a lamp or control the speed of a ventilation fan. This latter example is found particularly in television decoders. In fact, these electronic devices consume a great deal of energy during normal operation. This energy is transformed into heat that is concentrated within devices that are generally sealed. If it is not evacuated this heat provokes an accelerated aging of electronic components that results in irreversible deterioration. To prevent this, a ventilator is positioned close to air inlets in the casing of the decoder to accelerate exchanges with the exterior and improve cooling. But using a ventilator at full speed is noisy. If the device is placed in a room, the sound level can be disturbing. Experimentation has shown that it is not the speed that is audible but rather the variation in speed. A ventilator requires a minimal voltage to start, for example 5 volts for a maximum authorized voltage of 10 volts. If, a ventilator is controlled by a PWM signal, only digital values enabling generation of a voltage between 5 and 10 volts can be used. For an 8-bit PWM generator, these values are comprised between 128 and 255, that results in 128 possible values. As a result, the precision of such a device is 1:128 or 0.8% on the range of usable voltages. When the values are introduced into the PWM generator and a continuous voltage has thus been produced, the progression of one value to a next value is audible, especially when this variation intervenes regularly as is the case if a control system is used. In addition, different ventilator models are possible for a same device. If a servo system is used, the ventilators do not need to have the same characteristics, as a result the control system specific to the device must possess an extensive control range and a therefore a high level of accuracy over the entire range.
One solution consists in increasing the number of bits to program the PWM cyclic ratio, for example 12 bits, the precision is then at 0.025%. Assuming the same conditions as previously, the result is a precision of 0.05% that suits perfectly for reducing the audibility of the variations. But a 12-bit PWM is costly and takes longer to program than a PWM using values on 8 bits.
The document US 2007/098374—FUJIWARA published on May 3, 2007 describes a ventilator control system. A control unit 23 sends a PWM signal to the ventilator 22. A tachymetric probe detects the rotation speed of the ventilator and transmits it to the control unit 23. The system can control the temperature of a laptop computer. FIG. 7 of this document informs to use a frequency of 20 kHz for speeds greater than 5,000 rpm by applying a cyclic ratio varying from 70 to 100%, a frequency of 30 kHz for speeds between 4,000 and 5,000 rpm by applying a cyclic ratio varying from 50 to 70% and, a frequency of 40 kHz for speeds less than 4,000 rpm by applying a cyclic ratio varying from 25 to 50%. This document describes a direct correlation between the speed and the cyclic ratio value. The accuracy thus depends solely on that of the cyclic ratio.
The document U.S. Pat. No. 6,487,246—HOELD published on Nov. 26, 2002 describes the internal structure of a PWM signal generator. It is possible to separately program the number of clock pulses for the period and the number of clock pulses for the cyclic ratio of the PWM signal. This document specifies that if the programming of the two registers is carried out at the time of the generation of a new signal, this signal can be contain errors. The solution to this problem consists in synchronizing the update of the registers with the establishment of PWM periods, but this solution cannot increase the accuracy of the analogue signal generated.
The present invention enables a DC voltage to be generated from a more precise PWM signal without increasing the number of cyclic ratio programming bits.